Power supplies generally include regulated linear power supplies and switching power supplies. In a linear power supply, a control element, such as a series regulator transistor or variable resistor, is connected in series between a rectified supply voltage and a load. A feedback circuit monitors the output of the linear power supply and adjusts the series control element in response to maintain a constant output voltage Although transient response of a typical linear power supply is very quick, a linear power supply usually has low efficiency, since the series control element operating in the linear mode continuously dissipates power, and tends to generate a large amount of heat.
In comparison, a switching power supply typically operates at relatively high efficiency and low temperature rise. In a switching power supply, a pulse width modulated (PWM) element (or other circuit capable of rapid switching) is connected in series between a DC supply voltage and the load. A feedback circuit monitors the output of the switching power supply and controls the PWM circuit to vary the on/off periods (i.e., duty cycle) to control the output voltage during changes in output current. For example, in order to increase the output voltage, the PWM circuit may increase the width of the on pulse with respect to the width of the off pulse, resulting in an overall higher output voltage. Accordingly, switching power supplies dissipate much less power, and are smaller and lighter than comparable linear power supplies. However, a switching power supply typically has a slow transient response when responding to increased current demand, since it usually includes a relatively large output capacitor required to keep output ripple low.
Attempts to improve transient response in switching power supplies have generally included increasing the gain of the error amplifier, which detects changes in output voltage. For example, the value of a compensation capacitor of the error amplifier may be reduced in order to reduce the transient amplitude. In practice, however, this technique may result in an unstable or oscillatory control loop. Also, a conventional power supply described, for example, in U.S. Pat. No. 6,075,351, issued Jun. 1, 2000, which is hereby incorporated by reference, requires the power supply output stage itself to be capable of quick response when driven by its error amplifier. This solution is of limited value when attempting to speed up a switching power supply, for instance, which has inherently slow response due to the relatively large energy storage elements (e.g., inductors and capacitors).